Data storage devices such as disk drives provide data storage and retrieval in a variety of applications. A disk drive includes a spindle motor for rotating a disk, and a transducer head that moves radially across the disk to read from and write to concentric tracks on the disk. Many disk drives include multiple disks separated by spacer rings and stacked on a hub attached to the spindle motor, and multiple transducer heads that each read from and write to a different disk surface.
The tracks are each divided into circumferential divisions that are arced along the disk radius. The circumferential divisions each include a servo sector followed by a data sector. The servo sector contains servo information for positioning the transducer head over the track, and the data sector contains user data from an external device. The transducer head reads the servo sectors to position itself along the track as it reads and writes to and from the data sectors. In addition, the servo sectors are embedded in the tracks along servo wedges that extend radially across multiple tracks.
To access a data segment, the transducer head moves radially across the tracks to a destination track where the data segment starts during a seek operation. Thereafter, the disk rotates the start of the data segment on the track under the transducer head for reading or writing to or from the data segment during a track following operation. The data segment can continue onto one or more other tracks, in which case the transducer head sequentially moves to the subsequent tracks for accessing the remainder of the data segment.
The response time for accessing the data segment includes time periods for (1) moving the transducer head to the destination track (seek time), (2) rotating the start of the data segment under the transducer head (rotational latency time), and (3) recording or retrieving the data segment to or from the disk (transfer time). The access time is the seek time combined with the rotational latency time. Furthermore, the response time is inversely proportional to the data transfer rate (throughput). Thus, the access time is a significant performance feature since decreasing the access time decreases the response time which increases the data transfer rate of the disk drive.
FIG. 1 shows a conventional data segment layout in linear fashion. The data segments (DS) are each stored in two tracks (Tk) and contain a fixed number (x) of logical block addresses (LBA) per track. For instance, data segment DS0 is stored in tracks Tk0 and Tk1 and contains LBA0 to LBA2x, data segment DS1 is stored in tracks Tk2 and Tk3 and contains LBA2x+1 to LBA4x, and so on. The data segments have the same size, have the same number of LBAs, are arranged as sequential LBAs, occupy the same number of adjacent tracks, contain physically contiguous user data and fill the data sectors in the tracks they occupy.
The skew (phase advance or rotational skew angle) between adjacent (sequential) tracks as the disk rotates is a rational combination of the number of LBAs per track. The skew has a rotational latency time that is just greater than the seek time between adjacent tracks, and just greater than the head switch time between different transducer heads.
However, the data segments are not radially coherent. For instance, data segment DS1 has start and end rotational phases that are shifted relative to data segment DS0 by twice the skew, data segment DS2 has start and end rotational phases that are shifted relative to data segment DS1 by twice the skew and shifted relative to data segment DS0 by four times the skew, and so on.
Moreover, the intra-segment skew within a data segment is identical to the inter-segment skew between adjacent data segments. For instance, the intra-segment skew of data segment DS0 between tracks Tk0 and Tk1 is identical to the inter-segment skew of data segments DS0 and DS1 between tracks Tk1 and Tk2, the intra-segment skew of data segment DS1 between tracks Tk2 and Tk3 is identical to the inter-segment skew of data segments DS1 and DS2 between tracks Tk3 and Tk4, and so on.
Thus, the intra-segment skew between the start rotational phases of a data segment at adjacent tracks, and between the end rotational phases of a data segment at adjacent tracks, is the same as the inter-segment skew between the end rotational phase of a data segment and the start rotational phase of another data segment in adjacent tracks. Likewise, the inter-segment skew between the data segments varies as a function of the radial distance between the data segments.
The conventional data segment layout provides random access times for randomly selected data segments, which maximizes the forward sequential data transfer rate. As a result, the conventional data segment layout usually provides good data transfer rates when the disk drive supports computer applications. However, the conventional data segment layout provides poor data transfer rates when the disk drive supports consumer electronics applications with audio-video (AV) data (such as movies). For instance, the access time when moving sequentially backward through the AV data (as for reverse play or reverse search) is significantly higher than when moving forward through the AV data since the skew between the data segments is random and incoherent.
Disk drives typically perform the seek operation as fast as possible and then wait on average one-half a disk revolution for the start of the data segment to rotate under the transducer head (“hurry up and wait”). Unfortunately, the seek time and the rotational latency time are treated separately and combined by default although the access time is more important than the individual times. Furthermore, fast movements of the transducer head to reduce the seek time result in unwanted acoustic noise and high power consumption.
Disk drives have attempted to reduce the access time by altering the file system. However, the file system has incomplete knowledge of the data segment layout, and such knowledge can quickly become obsolete. As a result, the file system is over designed and creates cost penalties.
Disk drives have also attempted to reduce the access time by reordering the seek requests. Although seek request reordering can be effective when the disk drive supports computer applications, it is ineffective when the disk drive supports consumer electronics applications with multiple data streams of isochronous AV data since changing the order of requests results in failure to record or retrieve the correct AV data at the correct time.
There is, therefore, a need for improving access times and reducing acoustic noise in disk drives that store AV data.